A television signal receiver previously designed by one of the inventors, suitable for incorporation in a television set or video recorder, has a downconverter for responding to a selected television signal to generate intermediate frequencies including a downconverted amplitude-modulated video (or picture) carrier and a downconverted frequency-modulated audio (or sound) carrier. The downconverter includes a local oscillator for generating oscillations at a frequency controlled in response to an automatic fine tuning signal, which oscillations are heterodyned with the selected television signal to generate the intermediate frequencies. First and second output signals are taken from the downconverter by respective filters.
The filter producing the first output signal from the downconverter selects only the downconverted amplitude-modulated video carrier, suppressing the downconverted frequency-modulated audio carrier, and includes a deep adjacent-channel sound carrier trap. A first intermediate-frequency amplifier responds to the first downconverter output signal with a first-IF-amplifier response. A video detector generates a composite video signal responsive to the first-IF-amplifier response. This composite video signal is substantially free of sound beats therein, since the deep adjacent-channel sound carrier trap suppresses the adjacent-channel sound carrier in the IF supplied to the first IF amplifier. This first IF amplifier corresponds to the conventional video IF amplifier commonly referred to as a PIX IF amplifier.
In recent years, the level of the sound carrier has been reduced from the level in the original NTSC standard for television broadcasting, first by cable broadcasters to reduce-intrasystem interference and then by over-the-air broadcasters. This has tended to make the noise performance of intercarrier sound systems marginal, particularly in fringe-area reception of over-the-air television signals. The relatively deep adjacent-channel sound carrier trap for suppressing sound beats in the video detector response arising from an adjacent-channel sound carrier causes the first-IF-amplifier response to picture carrier to be reduced 6 dB or so, which results in reduced recovery of intercarrier sound intermediate frequencies therefrom. The peaking of the first-IF-amplifier response is for frequencies corresponding to midband video, so harmonic distortion of midband video signals generated in the intercarrier process are more likely to cause interference with an intercarrier sound IF signal supplied from the video detector to the FM sound detector, which interference disrupts proper limiting procedures, for example.
The pull-in range for the conventional automatic-fine-tuning (AFT) detector responding to the first-IF-amplifier response is designed to pull in from a mid-frequency of video IF, so the AFT will not erroneously lock to the adjacent-channel sound carrier. The deep adjacent-channel sound carrier rejection trap causes the first-IF-amplifier response to be sharply reduced with increasing frequency through the frequency region in which the downconverted sound carrier reposes, so the video carrier as translated to IF tends to be at a point in the IF amplifier response about -6 dB down from midchannel response. The reduced video carrier adversely affects the pull-in characteristics of the conventional automatic-fine-tuning (AFT) detector responding to the first-IF-amplifier response. Interfering carriers or sustained strong video signal components other than the video carrier, for which there are no rejection traps, tend to capture the AFT detector and cause mistuning. Using a separate intermediate-frequency amplifier for AFT is a possible way to obtain a better pull-in characteristics for the AFT detector, of course; but such practice ordinarily would be rejected in designing for the home market because of the additional receiver cost.
In the television signal receiver previously designed by one of the inventors, the filter producing the second output signal from the downconverter selects both the downconverted amplitude-modulated video carrier and the downconverted frequency-modulated audio carrier, and includes an adjacent-channel sound carrier trap that provides somewhat less rejection than the adjacent-channel sound carrier trap in the filter producing the first output signal from the downconverter and exhibits a less steep reduction with increasing frequency through the frequency region in which the downconverted sound carrier reposes. This somewhat less deep adjacent-channel sound carrier trap causes the second-IF-amplifier response to picture carrier to be reduced no more than 2 dB or so. The filter producing the second output signal from the downconverter in this previous design also includes a saddle between the video IF in-channel sound and picture carriers in which midband video IF is reduced by 10 dB or so, thereby eliminating the risk of doubled baseband video interfering with the sound IF signal.
Since the second-IF-amplifier response to picture carrier is reduced no more than 2 dB or so, the inventor attempted to obtain better pull-in characteristics for the AFT detector by using the second-IF-amplifier response as its input signal, rather than the first-IF-amplifier response. AFT detection is less affected by signals other than the video carrier, as desired, but the long-range pull-in characteristic on the low-frequency side is compromised if there is a saddle between the video IF in-channel sound and picture carriers in which midband video IF is reduced by 10 dB or so. Reducing the saddle so it is not more than 6 dB or so down reduces the likelihood of the AFT detector being captured by signals other than the video carrier without compromising its long-range pull-in characteristic on the low-frequency side. Even if the saddle is eliminated completely, the risk of doubled baseband video interfering with the sound IF signal is still lower than normally found in a PIX IF amplifier, since the video IF in-channel sound and picture carriers are stronger in the second-IF-amplifier response than in the first-IF-amplifier response. The benefits of both better sound and better AFT pull-in characteristics provide commercial justification for the cost of the second intermediate-frequency amplifier.